Heretofore, it has been known in the art to manufacture gas fired furnaces and heat exchangers utilizing stamped metal heat exchangers formed together in what has been commonly called, in the trade, a "clam shell" arrangement to create heat exchanger cells. These can be operated in either an upflow, a downflow, or a side flow arrangement, wherein hot gases of combustion are generated inside the clam shell, with the air or fluid medium to be heated being passed outside the clam shell so that heat exchange action takes place through the thin-walled, stamped metal of the cell. Typical of this type of furnace is one made by Tappan Air Conditioning and designated UG/UGI or the Janitrol Series 33 and 34 "Deluxe Upflow Furnace". These particular units utilize an aluminized/steel clam shell-type heat exchanger with a serpentine design for optimum efficiency of the stamped shell halves.
It is well known that this type of furnace design operates at about a 75 to 78 percent efficiency for conversion of the BTU energy of the input gas into heating capability for installation, the typical stack temperature at the immediate output of the heat exchangers is about 450.degree. F., with this being diluted with room air to create a positive stack pressure so that at about two or three feet up, the stack temperature is about 350.degree. F. In order to create a positive stack flow, a room air vent is conventional for this type of furnace.
The problem with this design is that nearly 25 percent of the potential energy of the gas is really lost as exhaust gases up the stack. In addition, the fact that these furnaces operate on the basis of positive venting which requires a room air vent (technically referred to as an "integral draft diverter") in association with the stack draws some of the heated room air up the stack, further decreasing the efficiency of the unit.